1. Field of the Invention
The present invention relates to a semiconductor laser device.
2. Background Art
Semiconductor laser devices configured to have a relatively narrow far field pattern (FFP) have been known, as disclosed, e.g., in Japanese Laid-Open Patent Publication No. H11-233882. Specifically, this publication discloses a semiconductor laser in which an n-side (or first conductivity type side) low refractive index layer is provided between an n-type cladding layer and an n-side light guiding layer, with the n-side low refractive index layer having a lower refractive index than the n-type cladding layer. This configuration allows for widening the optical intensity distribution in the crystal growth direction and thereby narrowing the far field pattern (FFP) of the semiconductor laser.
Other prior art includes Japanese Unexamined Patent Application Publication No. H11-233882, Japanese Laid-Open Patent Publication No. 2000-151018, Japanese Laid-Open Patent Publication No. 2007-220692, Japanese Laid-Open Patent Publication No. H8-195529.
Other prior art includes a non-patent publication M. Alam and M. Lundstrom, “Simple Analysis of Carrier Transport and Buildup in Separate Confinement Heterostructure Quantum Well Lasers”, IEEE, Pnotonics Tecnol. Lett., Vol. 6, No. 12, pp. 1418-1420, 1994. Other prior art also includes a non-patent publication Iga, “Semiconductor Laser”, pp. 35-38, Oct. 25, 1994, Ohmsha, Ltd.
However, the above conventional semiconductor laser device is disadvantageous in that the width of the near field pattern (NFP), or near field intensity profile, of the laser beam as measured in the crystal growth direction is relatively large, since the low refractive index layer having a lower refractive index than the n-type cladding layer is provided between the n-type cladding layer and the n-side light guiding layer. This reduces the optical confinement factor in the active layer, resulting in an increased threshold current of the semiconductor laser device.
In conventional semiconductor laser devices, the active layer may be displaced from the lateral center plane of the light guiding layer structure (which is formed of the n-side and p-side light guiding layers with the active layer sandwiched therebetween) toward the p-type cladding layer in order to improve the slope efficiency. This configuration, however, results in an increased threshold current of the semiconductor laser device.